Hello, Buz has the answer by dampening the wood slightly. I might add...that if it was a deep dent, the "surface can be dry.... but underneath is still not cured. I know because I had the same thing happen to me once. I also have a suggestion when storing the balsa filler. I have a water bottle with a VERY fine mist that I use to sometimes spray on balsa for various reasons. BEFORE closing up my balsa filler I give the contents a "short blast" of water mist. I also spray a little on the inside of the lid. The idea here is to keep moisture in the container to keep the filler from drying out. Just don't OVER DO it, as you can weaken the mixture with too much water. I also have had problems smoothing a surface sometimes by messing around too long. Again.... a quick, short, blast of water mist will help the spatchula glide over the filler and keep it from "pulling up " imperfection etc. Again, be very careful not to get it wet!!
Good luck......lownslo